From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 262C8D0C84E for ; Tue, 13 Jan 2026 10:49:47 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vfby1-0008JV-71; Tue, 13 Jan 2026 05:49:42 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vfbxo-0008GJ-1H for qemu-rust@nongnu.org; Tue, 13 Jan 2026 05:49:28 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vfbxm-0004wI-0W for qemu-rust@nongnu.org; Tue, 13 Jan 2026 05:49:27 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1768301364; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Fun+VlAGuTbg8mclMhkVXEMkwi0/yRlPk1nArL251uk=; b=Q1r/HEIqvyiQmMLT5g9YxLKAjSCXZIf6B0cS3sIl6EuTQUfz6MBgHFmi5xDC/voiPuBdo1 xmYNjSlPZt/97FZpcdlvZvPUdVBhTOUqkieHrnsEkMDei4bmxRkvWBC9xCZu8B+SAVJGw4 +N6FVMS07p3FYxf3kjK33BWRjbrHLUo= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-131-52zlFBwbNcan1pWjRbTeRg-1; Tue, 13 Jan 2026 05:49:21 -0500 X-MC-Unique: 52zlFBwbNcan1pWjRbTeRg-1 X-Mimecast-MFC-AGG-ID: 52zlFBwbNcan1pWjRbTeRg_1768301359 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 842B71956089; Tue, 13 Jan 2026 10:49:19 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.87]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 683331800665; Tue, 13 Jan 2026 10:49:06 +0000 (UTC) Date: Tue, 13 Jan 2026 10:49:01 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster Cc: qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Manos Pitsidianakis , Hanna Reitz , Gerd Hoffmann , Paolo Bonzini , Christian Schoenebeck , "Dr. David Alan Gilbert" , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , devel@lists.libvirt.org, qemu-block@nongnu.org, qemu-rust@nongnu.org, Stefan Weil , Kevin Wolf , Richard Henderson Subject: Re: [PATCH v5 09/24] util: introduce some API docs for logging APIs Message-ID: References: <20260108170338.2693853-1-berrange@redhat.com> <20260108170338.2693853-10-berrange@redhat.com> <87ms2hal4a.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87ms2hal4a.fsf@pond.sub.org> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-rust@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: QEMU Rust-related patches and discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org Sender: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org On Tue, Jan 13, 2026 at 10:59:49AM +0100, Markus Armbruster wrote: > Daniel P. Berrangé writes: > > > There is a gotcha with qemu_log() usage in a threaded process. > > If fragments of a log message are output via qemu_log() it is > > possible for messages from two threads to get mixed up. To > > prevent this qemu_log_trylock() should be used, along with > > fprintf(f) calls. > > > > This is a subtle problem that needs to be explained in the > > API docs to ensure correct usage. > > > > Reported-by: Markus Armbruster > > Signed-off-by: Daniel P. Berrangé > > --- > > include/qemu/log-for-trace.h | 17 ++++++++++++++++- > > include/qemu/log.h | 31 +++++++++++++++++++++++++++++++ > > rust/util/src/log.rs | 6 ++++++ > > 3 files changed, 53 insertions(+), 1 deletion(-) > > > > diff --git a/include/qemu/log-for-trace.h b/include/qemu/log-for-trace.h > > index f3a8791f1d..6861a1a4b7 100644 > > --- a/include/qemu/log-for-trace.h > > +++ b/include/qemu/log-for-trace.h > > @@ -29,7 +29,22 @@ static inline bool qemu_loglevel_mask(int mask) > > return (qemu_loglevel & mask) != 0; > > } > > > > -/* main logging function */ > > +/** > > + * qemu_log: report a log message > > + * @fmt: the format string for the message > > + * @...: the format string arguments > > + * > > + * This will emit a log message to the current output stream. > > + * > > + * The @fmt string should normally represent a complete line > > + * of text, and thus end with a newline character. > > Note for later: "should normally". > > > + * > > + * While it is possible to incrementally output fragments of > > + * a complete line using qemu_log, this is inefficient and > > + * races with other threads. For outputting fragments it is > > + * strongly preferred to use the qemu_log_trylock() method > > + * combined with fprintf(). > > + */ > > void G_GNUC_PRINTF(1, 2) qemu_log(const char *fmt, ...); > > > > #endif > > diff --git a/include/qemu/log.h b/include/qemu/log.h > > index 7effba4da4..e9d3c6806b 100644 > > --- a/include/qemu/log.h > > +++ b/include/qemu/log.h > > @@ -41,7 +41,38 @@ bool qemu_log_separate(void); > > > > /* Lock/unlock output. */ > > > > +/** > > + * Acquires a lock on the current log output stream. > > + * The returned FILE object should be used with the > > + * fprintf() function to output the log message, and > > + * then qemu_log_unlock() called to release the lock. > > + * > > + * The primary use case is to be able to incrementally > > + * output fragments of a complete log message in an > > + * efficient and race free manner. > > + * > > + * The simpler qemu_log() method must only be used > > + * to output complete log messages. > > "must". > > > + * > > + * A typical usage pattern would be > > + * > > + * FILE *f = qemu_log_trylock() > > + * > > + * fprintf(f, "Something "); > > + * fprintf(f, "Something "); > > + * fprintf(f, "Something "); > > + * fprintf(f, "The end\n"); > > + * > > + * qemu_log_unlock(f); > > + * > > + * Returns: the current FILE if available, NULL on error > > + */ > > FILE *qemu_log_trylock(void) G_GNUC_WARN_UNUSED_RESULT; > > + > > +/** > > + * Releases the lock on the log output, previously > > + * acquired by qemu_log_trylock(). > > + */ > > void qemu_log_unlock(FILE *fd); > > > > /* Logging functions: */ > > diff --git a/rust/util/src/log.rs b/rust/util/src/log.rs > > index 0a4bc4249a..6a3a30d8d8 100644 > > --- a/rust/util/src/log.rs > > +++ b/rust/util/src/log.rs > > @@ -134,6 +134,12 @@ fn drop(&mut self) { > > /// "Address 0x{:x} out of range", > > /// error_address, > > /// ); > > +/// > > +/// The `log_mask_ln` macro should only be used for emitting complete > > "should only". > > > +/// log messages. Where it is required to incrementally output string > > +/// fragments to construct a complete message, `LogGuard::new()` should > > +/// be directly used in combination with `writeln()` to avoid output > > +/// races with other QEMU threads. > > /// ``` > > #[macro_export] > > macro_rules! log_mask_ln { > > "Should normally" suggests there are exceptions. > > "Should only" does not. > > "Must" is a bit stronger still. > > Which of the three do we want? The "Should" usage was reflecting the reality that we have quite alot of code using qemu_log_trylock + qemu_log, instead of qemu_log_trylock + fprintf. I didn't feel it appropriate to use 'must' unless we explicitly make qemu_log() fail when used inside the scope of a qemu_log_trylock(). So "Should normally" is the best fit With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|